Polarizing plates (also commonly called “polarizers”) are widely used in displays to control incoming and outgoing light. For example, in LCDs, a liquid crystal cell is typically situated between a pair of polarizer plates. Incident light polarized by the first polarizer plate passes through a liquid crystal cell and is affected by the molecular orientation of the liquid crystal, which can be altered by the application of a voltage across the cell. The altered light goes into the second polarizer plate. By employing this principle, the transmission of light from an external source, including ambient light, can be controlled. LCD's are quickly taking over from traditional CRTs in computer monitors and TVs because of their lower energy consumption and thinness.
The principle of OLED Display operation does not rely on the polarization state of light. In OLED Displays, organic compounds emit light as a current goes through, thus, no modulation of polarization is involved. However, in order to reject ambient light, OLED displays use a combination of an optical retardation film and a polarizer, such as a circular polarizer plate. In display applications including LCD and OLED, it is important that the degree of polarization, so called polarization efficiency, of the light through the polarizer plate is high. A low degree of polarization would result in inferior display performance.
Most of the polarizer plates currently used in displays are transmission-absorptive type. They polarize incident natural light by absorbing a component of the electric field oscillating in a particular direction. The transmitted light is linearly polarized. In order to generate the absorption-dichromatic polarizing element that functions over a wide range of visible spectrum, a sheet of polyvinyl alcohol (PVA) impregnated with iodine or organic dichroic dyes is stretched to produce a polarizing film. In most cases the absorption axis is parallel to the direction of stretch. Polarizing films composed of said oriented PVA sheet alone do not possess good humidity or temperature stability. Thus, they are sandwiched between two protective layers 101, 103, FIG. 1, to form a polarizing plate. Typically the protective layers are made from cellulose triacetate (TAC). The incoming un-polarized light 105 transmits through the protective layer 101, and is polarized through the polarizing film element 107. The linearly polarized light 109 comes out from the opposite side through the other protective layer 103. The protective layers must satisfy certain quality requirements: high transparency, low birefringence, dimensional stability against temperature and humidity, and chemical stability.
For display applications, overall light efficiency is an important factor for performance. Thus the protective film should be free from haze, high absorption, and coloration. Preferably, transmission of the protective film is above 90%. In LCDs, the polarization state of the light is modulated by changing the birefringence of the liquid crystal cell and additional birefringence contribution from polarizer protective layers often has to be suppressed. Also, a low birefringence protective layer enables efficient manufacturing of various combinations of polarizer and retarders.
Long-term reliability of the polarizer plates depends on two factors: chemical and dimensional stability. The function of a polarizing element 107 relies on the alignment and the chemical state of iodine or dichroic dyes embedded within it. In dichroic dye type polarizers, the desired absorption spectrum is given by a mixture of dichroic dyes possessing overlapping absorption bands to cover the intended useful light wavelength range. In iodine type polarizer films, the desired absorption spectrum is given by controlling the amount of poly-iodide ions, I3− and I5−. The degree of polarization is directly related to the degree of alignment of these ionic species. If the ionic state or alignment is degraded, the polarization degree of light would not be satisfactory for display applications. Therefore, the protective layers have to be free from chemical species that can react with or diffuse into the polarizing element and alter the state of poly-ions or dyes.
Dimensional instability of the protecting layer(s) (101, 103) degrades the polarizer plate performance mainly by two mechanisms: photo elasticity and change in the local polarization direction. Due to the effect of photo elasticity, a protective layer would generate undesired birefringence on its own when it shrinks or expands due to temperature or humidity. With the birefringence in the protective layer, the linearly polarized light 109 exiting the polarizing element 107 will be altered to elliptically polarized light while it transmits through the second protective layer 103. In most cases, such a birefringence caused by photo elasticity is not uniform across the plane of the polarizer and thus creates an inhomogeneous brightness appearance in the display application. Even if the protective film has no photo elasticity, dimensional change can alter the local polarization direction as it is adhered to the polarizing element typically made from a film of polyvinyl alcohol.
Intrinsic birefringence describes the fundamental orientation of a material at a molecular level. It is directly related to the molecular structure (bond angles, rotational freedom, presence of aromatic groups, etc.) of the material. The intrinsic birefringence is not affected by process conditions (temperature, stresses, pressures) used to make a macroscopic object.
Crystalline and liquid crystalline materials have the convenient property that their intrinsic birefringence manifests itself almost perfectly when they are assembled into a macroscopic article. Layers of crystalline and liquid crystalline molecules often can be manufactured such that a preponderance of the molecules in the article are in registry with each other and thus preserve their fundamental orientation. The same is not true when making layers of an amorphous polymeric material. Their intrinsic birefringence can be highly modified by the manufacturing process. Thus, the measured birefringence of an actual article will be a resultant of its intrinsic birefringence and the manufacturing process. Because we are dealing with such amorphous polymeric materials, the following definitions refer to this measured birefringence and not intrinsic birefringence.
In-plane birefringence means the difference between nx and ny, where x and y lie in the plane of the layer. nx will be defined as being parallel to the casting direction of the polymer, and ny being perpendicular to the casting direction of the polymer film. The sign convention used will be nx−ny.
Out of-plane birefringence means the difference between nz and the average of nx and ny, where x and y lie in the plane of the layer and z lies in the plane normal to the layer. The sign convention used will be: nz−[(nx+ny)/2]. TAC typically has a negative out of plane birefringence as its nz is smaller than its nx and ny.
In-plane retardation (Re) means the product of in-plane birefringence and layer thickness (t). Thus Re=t(nx−ny)
Out-of-plane retardation (Rth) means the product of out-of-plane birefringence and layer thickness (t). Thus Rth=t(nz−[(nx+ny)/2]).
TAC film has been widely used as the protective layers of polarizer plates. It is relatively simple to obtain low birefringence, especially in-plane, by adjustment of the casting process. This is due to a low intrinsic birefringence of TAC. By carefully choosing the dopant material to TAC (e.g., UV absorber, plasticizers), it is possible to prevent the degradation of the polarizing element (dyed PVA film) by chemical processes. Good optical properties (low haze, relatively high transmission) and its low cost make TAC attractive for this application.
Although TAC satisfies the essential requirements of the protective layers for polarizer plates, it has problems in birefringence and dimensional stability.
As mentioned above, it is possible to lower the birefringence of TAC by changing the casting conditions. This holds true for in-plane birefringence and it is typically 3×10−5. However, there is typically residual out-of-plane birefringence on the order of 6×10−4. Thus, TAC film with a thickness of 100 μm would have low in-plane retardation of 3 nm but the out-of-plane retardation (Rth) would be 60 nm. Such a high value of Rth is often a major concern in display applications. The generation of higher out-of-plane retardation is due to the stress generated in the film normal direction during the drying of solvent used during casting. A few modes of LCD utilize this residual Rth to enhance the viewing quality of LCDs. In some applications, it is preferable to have a much lower Rth contribution from the protective layer.
TAC can have high residual stress generated during the solvent casting process. This often contributes to its low dimensional stability. A dimension change in TAC, such as shrinkage, leads to a local change in the polarization direction or lowering of the degree of polarization.
Another problem with TAC is its high moisture permeability. TAC is generally adhered to polarizing film elements with a water soluble adhesion agent after saponification treatment of the TAC film surface. Thus, such humidity permeation leads to a delamination between the TAC and polarizing film element. Also, the ionic state of the dichroic components in the polarizing film element changes state in the presence of moisture leading to a lower degree of polarization of the outcoming light.
An additional complication in the polarizer plate manufacturing process is the saponification treatment required to prepare the TAC film surface for adhesion to the PVA polarizing film element. Since this involves treatment with strong alkaline solution, it raises environmental and working safety concerns, as well as being a source of protective layer non-uniformity defects. Such a strong alkaline treatment combined with the use of plasticizer and other dopant in TAC, can result in segregation, so called bleeding problems. Therefore, it is desirable to eliminate the need for saponification treatment from the polarizer plate manufacturing process.
JP 2001-206981 discloses a process to manufacture cellulose ester film having low Rth. The process involves dissolving cellulose ester into a solvent containing bromopropane. As bromopropane has a higher boiling point than the more generally used solvent, methylene chloride, the slow evaporation process is intended to relax the polymer chain. Reduction in the Rth of TAC from 85 nm to 25˜40 nm was claimed. However, the residual Rth is still significant and the use of such a solvent poses a significant potential health hazard.
Other well-known polymeric materials having low birefringence are polynorbornene type polymers. Films of such polymers made by solvent casting or extrusion processes usually shows a few nm in Rth. Not only do such polymer films have low birefringence, also they exhibit low moisture permeation and high dimensional stability. U.S. Pat. No. 6,552,145 discloses use of such a polymer as a protective layer for a polarizer plate. Though various desirable characteristics are shown, the cost of such a film is generally much higher than TAC. Also, adhesion of such polymer layers to PVA is a significant challenge as compared to that of cellulose ester films. JP2001-215331 teaches art in which a protective layer film is comprised of a core layer and a layer of cellulose derivative disposed on at least one side of the core layer. By having the layer of cellulose derivative, adhesion to PVA can be improved. However, such an extra coating process only increases the manufacturing cost of polarizer protective layers.
Therefore, there is a need for a new protective layer for polarizing plates having low in-plane and out-of-plane birefringence and higher dimensional stability than TAC. Yet another need exists for a process to provide such a protective layer for polarizing elements that is free from the need to use strong alkaline solutions, that pose environmental and work-safety concerns.